try harder to find Jupyter and Python during build

[stevengj]

This PR tries harder to correctly configure Interact on Windows.

On Windows, running jupyter subcommands can be tricky because of JuliaLang/IJulia.jl#363. I had to reproduce some of the logic from the IJulia build script here.

Also, it tries a bit harder on Windows to find python.exe, especially for Conda builds.

This also fixes the issue with @PYTHON_str mentioned in #205.

[stevengj]

cc @alanedelman, hopefully this fixes the problem your student was having.

[alanedelman]

Thanks!

[stevengj]

@shash, the CI failure on nightly seems unrelated ...

[shashi]

This looks good. Thanks!!

[stevengj]

Alan, you may want to post on Piazza that students who had trouble getting @manipulate to work when they installed their own Julia on Windows should try: Pkg.checkout(“Interact”); Pkg.build(“Interact”) to get the latest version.